Server-side timer synchronized with Client

I’m building an application that allows users to submit images, after a
certain period of time (based on the number of current users) the
highest rated image is selected and the application will begin accepting
a new set of submissions.

My question is, how should I implement this? Should I use threads, or
what other options do I have? Also, what kind of scheme could I use to
synchronize the timer with all of the clients (i.e. an accurate timer on
the page)?

Thanks in advance,
Pat